For players new to roguelikes, I'd also have to recommend DoomRL (http://doom.chaosforge.org/) as a fantastic game, and its shorter length might appeal to more casual players.
ADOM is one of the best roguelikes. It's closed source but very deep and high quality. http://adom.de/
POWDER is a little graphical roguelike targetted at portable game consoles and PDAs. But you can play it from PC. It's reasonably quick and have a very fun and orthogonal skill/spell/item system. http://www.zincland.com/powder/index.php?pagename=release
Just today the team took a patch of mine to simplify poisoning!
Graphical, short and really easy to get into. Runs on a multitude of platforms, also GBA and other consoles.